The ability to identify the presence of an intruder is important in a variety of contexts: military surveillance, commercial security, and domestic security. A number of approaches are presently employed to identify the presence of an intruder on secured premises.
Cameras are widely used to provide surveillance of a controlled area. The problem with this approach is that it is relatively expensive. In addition, the camera is typically placed in a position which can be identified by the intruder. As a result, the intruder can take evasive action to avoid detection by the camera. Yet another problem associated with this approach is that an individual must visually monitor the output of the camera at all times. A camera will not produce an audio output to indicate the presence of an intruder. Still another problem with this approach is that a camera placed outdoors is subject to harsh physical forces such as heat, rain, and cold. Another limit with camera based systems is that they must be coupled with a lighting system when used at nighttime or within a building.
Another approach to detecting intruders is to use an active antenna-based system. In this system, an antenna radiates a signal. Disruptions in the radiated signal can indicate the presence of an intruder. Since these active systems radiate signals, they are power-consuming. Another disadvantage of these devices is that an intruder may detect the radiated signal. In addition, these devices utilize elaborate circuitry and consequently are expensive to manufacture. Another problem is that they tend to drift with time and temperature. Finally, these devices may be triggered by stray electromagnetic fields.
Electrical discharge plates have been used in the prior art to detect the presence of an intruder. In such a system, a discharge plate is used as a sensor electrode which is triggered when an intruder is immediately adjacent to the discharge plate. The shortcomings of this approach are obvious--the limited range of the plates necessitate a high density of plates in any given area. Another problem associated with this approach is that it does not provide any information on the character of the intruder, for instance, the physical size of the intruder or the distance.